One important aim in the development of power semiconductor devices, such as power transistors or power diodes, is to produce devices with a high voltage blocking capability but, nevertheless, a low on-resistance (RON) in case of a transistor and a low forward voltage drop in case of a diode. Further, it is desired to have low losses when the power semiconductor device changes between an on-state (blocking state) and an off-state (conducting state).
Power transistors usually include a drift region arranged between a body region and a drain region and doped lower than the drain region. The on-resistance of a conventional power transistor is dependent on the length of the drift region in a current flow direction and on the doping concentration of the drift region, wherein the on-resistance decreases when the length of the drift region is reduced or when the doping concentration in the drift region is increased. However, reducing the length of the region or increasing the doping concentration reduces the voltage blocking capability.
One possible way to reduce the on-resistance of a power transistor having a given voltage blocking capability is to provide compensation regions in the drift region, wherein the compensation regions are doped complementary to the drift region. Another possible way is to provide field plates in the drift region which are dielectrically insulated from the drift region and which are, for example, connected to a gate or source terminal of the transistor. In these types of power transistors, the compensation zones or the field plates partly “compensate” doping charges in the drift region when the component is in its off-state. This allows to provide a higher doping of the drift region—which reduces the on-resistance—without reducing the voltage blocking capability.
A power diode (pin diode) usually includes a low doped drift or base region between a first emitter region of a first doping type and a second emitter region of a second doping type. A power diode blocks when a voltage with a first polarity (blocking voltage) is applied between the first and second emitter regions, and conducts when a voltage with a second polarity is applied between the first and second emitter regions. In the conducting state, however, a charge carrier plasma with charge carriers of the first and second type (p-type and n-type charge carriers) is generated in the base region. The amount of charge carrier plasma stored in the base region is dependent on a length of the base region and is, therefore, dependent on the voltage blocking capability, where the amount of charge carrier plasma increases when the voltage blocking capability increases. This charge carrier plasma has to be removed before the diode may block upon applying a blocking voltage.
Recently a new type of power semiconductor device referred to as ADR (Active Drift Region) device or ADZ (Active Drift Zone) device has been proposed. An ADR device such as an ADRFET (Active Drift Region Field-Effect Transistor) or an ADR diode includes a first semiconductor device such as a transistor or a diode, and a plurality of second semiconductor device such as transistors connected in series with the first semiconductor device. The second semiconductor devices form the Active Drift Region of the device and are interconnected such that the operation states of the second semiconductor devices follow the operation state of the first semiconductor device. That is, the second semiconductor devices are conducting when the first semiconductor device is conducting, and the second semiconductor devices are blocking when the first semiconductor device is blocking. The overall voltage blocking capability of the power semiconductor device corresponds to the sum of the voltage blocking capabilities of the individual second semiconductor devices. That is, the individual second transistors share the overall blocking voltage applied to the power semiconductor device.
According to one approach, the second semiconductor devices in an ADR device are implemented as normally-on transistors such as depletion MOSFETs (Metal Oxide Field-Effect Transistors) or JFETs (Junction Field Effect Transistors). Each of these normally on-transistors has a control terminal (gate terminal) connected to the load terminal (drain or source terminal) of one of the other second transistors. Depletion MOSFETs or JFETs are voltage controlled devices that can be switched on and off through a control voltage applied between the control terminal and one of the load terminals. When the second transistors are interconnected as explained above, the control voltage of one second semiconductor device corresponds to the load path voltage of at least one other second semiconductor device. This is usually noncritical when the second semiconductor devices are implemented with a low voltage blocking capability such as a voltage blocking capability of between 5V and 20V. In these devices, the maximum voltage that can be applied as a control voltage is in the same voltage range.
However, it may desirable to use second semiconductor devices with higher blocking voltages such as blocking voltages of up to several 10V or even up to several 100V in order to provide a power semiconductor device with a voltage blocking capability of several kilovolts (kV). Usually, these semiconductor devices are not capable of withstanding control voltages that are as high as several 10V or even several 100V.
There is therefore a need to provide a semiconductor device arrangement with a first semiconductor device and with a plurality of second semiconductor devices that is implemented with second semiconductor devices having high blocking voltages.